Angiogenesis inhibitors are being tested as cancer therapeutics with the idea of controlling tumor growth by preventing expansion of their blood supply, but little is known about the intermediary mechanisms and processes involved. Study of the effects of one inhibitor, rIL-12, within treated tumors reveals that a mechanism by which it controls tumor growth involves induction of severe tumor cell hypoxia and hypoxia-induced apoptosis. To establish whether angiogenesis inhibition leading to tumor hypoxia and hypoxia-induced apoptosis is a general physiological pathway invoked by these agents, Aim 1 is to examine the effect of selected antiangiogenic agents on tumor physiology. Effects of endostatin, thalidomide, thalidomide analogs and other putative angiogenesis inhibitors on tumor cell hypoxia, proliferation and apoptosis and on tumor vessel localization, density and patency will be examined. These studies will elucidate tumor mechanisms activated by antiangiogenesis therapy to control tumor growth. While induction of tumor hypoxia may be important for the therapeutic efficacy of angiogenesis inhibitors, it is also an environmental stress that can exert selective pressures on tumor cells. In Aim 2, the effect of iatrogenic hypoxia on cell selection in tumors will be examined by seeing if it results in the outgrowth of tumor cells engineered to better survive hypoxia. Other studies will examine tumors treated with antiangiogenic agents to see if cells emerge that resist hypoxia- induced apoptosis. These studies will indicate whether inhibitor therapy of tumors carries the potential risk of selecting tumor cells with diminished apoptotic potential. Angiogenesis inhibitors undoubtedly will be considered for use with radiation therapy. While tumor response to radiation may be improved by angiogenesis inhibition since both reduce and restrict tumor growth, the hypoxia that results from inhibitor use may engender radiation resistance. In Aim 3, the interaction between angiogenesis inhibitor and radiation therapy will be examined by studying the effect of radiation therapy alone and radiation given with antiangiogenic agents before and after they have induced tumor hypoxia. Measurement of tumor cell clonagenic survival, tumor remission, and the tumor physiological effects of therapy will detail the interaction between radiation and angiogenesis inhibitors in the presence and absence of hypoxia. Favorable and/or antagonistic interactions revealed by these studies can guide their combined clinical use and should elucidate the role of angiogenesis and tumor cell killing in tumor response to radiation therapy.